Improved process for the manufacture of methyl acrylate by the thermal decomposition of methyl alpha-acetoxypropionate



Patented Sept. 24, 1946 IMPROVED PROCESS FOR THE MANUFAC- TURE OF METHYL ACRYLATE BY THE THERMAL DECOMPOSITION OF METHYL ALPHA-ACETOXYPROPIONATE William P. Ratchford, Willow Grove, and Charles r -H. Fisher, Abington, Pa., assignors to Claude R. Wickard, as Secretary of Agriculture of the United States of America, and his successors in No Drawing. Application May 4, 1943, Serial No. 485,618

1 Claim.

(Granted under the act of March 3, 1883, as amended April 30, 1928; 3'70 0. G. 757) This application is made under the act of March 3, 1883, as amended by the act of April 30, 1928, and the invention herein described, if patented, may be manufactured and used by or for the Government of the United States of America for governmental purposes without the payment to us of any royalty thereon.

Our invention relates to the manufacture of unsaturated organic compounds and more particularly to the manufacture of acrylic esters by lysis generally yield the carboxylic acid and an unsaturated compound (Pelt and Wibaut, Rec.

trav. chem. 60, 55-64 (1941)) Hurd, Pyrolysis f Carbon Compounds, Reinhold Publishing Corp., New York city (1929). 7

An object of our invention is to provide a commercial process for manufacturing unsaturated compounds, suchas acrylic esters, by passing certain esters of carboxylic acids through a hot tube or chamber.

A further object is to provide improved conditions for pyrolysis which will increase the rate of conversion, increase the capacity of a pyrolysis unit of any given size, and decrease production costs and capital investment.

Other objects of our invention will be apparent from the following description.

We have found that esters of carboxylic acids can be transformed into unsaturated compounds by pyrolysis under pressures much higher than atmospheric pressure and that certain lactic acid derivatives can be converted into acrylic esters by pyrolysis of lactic acid derivatives under increased pressures. Moreover, we have found that methyl alpha-acetoirypropionate, which is prepared conveniently by acetylating the methyl ester of lactic acid, can be pyrolyzed satisfactorily at high pressures and that methyl acrylate is produced in good yield by this operation. We have also found that the pyrolysis of mixtures of methyl lactate and acetic anhydride under superatmospheric pressures yields methyl acrylate.

Further, we have found that the throughput of any given pyrolysis unit, and hence its capacthe pyrolysis under pressure. In addition, the

use of superatmospheric pressures as herein described hasthe following advantages over operation at essentially atmospheric pressure: (1) Better heat transfer is afforded, and (2) the boiling points of the products are raised and hence condensation of the products is facilitated.

The pyrolysis chamber, may be empty or may contain packing materials such as quartz chips, Raschig'rings, lumps of coke, metal shavings, and so forth. Vapors of the carboXylic acid esters may be passed in at the bottom of the chamher and out at the top, or in at the top and out at the bottom, or if desired, the pyrolysis tube may be mounted in a horizontal position.

In the examples set forth below, illustrating our invention, the reagent, methyl alpha-acetoxypropionate, was allowed to flow by gravity from a calibrated cylinder into a constant-feed pump capable of operating under high pressures. The reagent was pumped under pressure into heated stainless-steel tubing inside diameter which was heated over a length of approximately 12 feet. The temperature was essentially uniform over a considerable length of the heated 12-foot section, although some of the forward end of the tubing served as a preheater. After passing through the heated reaction zone, the vapors passed through a cooled condenser and into a high-pressure receiver. Gases were released through a throttling valve from the top of this receiver at such a rate that the desired pressure was maintained in the system- The liquid products were collected at the bottom of the receiver -by manipulation of a suitable Valve.

Example I Methyl alpha-acetoxypropionate (315.5 grams) was pumped into the pyrolysis chamber at the rate of 1.12 moles per hour. The reaction temperature and pressure were 550 C. and 45 pounds per square inch, respectively. The liquid products were 85.4 percent of the charge. High conversions into methyl acrylate and acetic acid were obtained.

Example I 1 Methyl alpha-acetoxypropionate (858 grams) was pumped at pounds per square inch into the pyrolysis chamber at 550 C. The rate of pumping was 3.18 moles per hour. High conversions into methyl acrylate and acetic acid w re obtained.

Example III Methyl alpha-acetoxypropionate (1580 grams) was pumped at 900 pounds per square inch into the pyrolysis chamber at 488 C. at a pumping rate of 23 moles per hour. The reagent was satisfactorily converted into methyl acrylate and acetic acid.

Our invention is not limited to the examples given above, but is applicable generally to the 10 

